Here is more explanation of the symbolism of the Donkey:
http://ravkooktorah.org/BO_67.htmBo: Donkey-HolinessImmediately before leaving Egypt, the Israelites were commanded to commemorate the final plague of makkat bechorot, the death of the first-born, by consecrating the firstborn:
"When Pharaoh stubbornly refused to let us leave, G-d killed all the first-born in Egypt, both man and beast. I therefore offer to G-d all male firstling animals, and redeem all the first-born of my sons." (Ex. 13:15)
This mitzvah applies not only to people, but also to kosher animals, and — surprisingly — first-born donkeys: "Every firstling donkey must be redeemed with a sheep" (Ex. 13:13).
Why does this holiness of the firstborn apply to donkeys?
The firstborn holiness of donkeys is even more surprising when we consider that these animals are 100% impure. Some non-kosher animals, such as camels and pigs, have only one sign of impurity. Donkeys, however, carry both signs of impurity. The Zohar teaches that the donkey is avi avot hatumah, the ultimate source of impurity.
In addition, 16th century Rabbi Judah Loew ben Bezalel, the Maharal of Prague, noted that the Hebrew word for donkey (chamur) is the same as the word for material (chomer). The donkey, he explained, is a symbol of crassness and physicality (Gevurot Hashem, ch. 29).
So why did the Torah designate this ignoble creature to have the special holiness of bechor that must be redeemed?
Hidden Holiness
One explanation proposed by the Sages in Bechorot 5b is that the donkeys helped facilitate the Exodus as they carried the treasures of Egyptian gold and silver. Yet the Israelites could have used some other pack animal. It would appear that there is something special about the donkey, that it represents an inner truth about the redemption of the Jewish people, both in Egypt and in the future national rebirth of the Messianic Era.The Israelites in Egypt had sunk to the lowest levels of immorality and impurity. Outwardly, they were indistinguishable from their Egyptian masters. Even the angels were unable to distinguish between the two nations. They questioned G-d's decision to save the Israelites at the Red Sea, protesting, "These are idol worshippers and these are idol worshippers!"
But like the donkey, the impurity of the Jewish people was only on the surface, hiding a great inner holiness. It was a superficial blemish, as it says, "Do not look upon me that I am black; for (it is only) the sun that has tanned me" (Song of Songs 1:6).
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